HetNet Mobility Management

ABSTRACT

A wireless communication system is presented for robust mobility management in a HetNet communication system. A source cell can prepare a macro cell and a target small cell as handover candidates during handover decision making and/or preparation. The mobile device is informed about the prepared macro cell and target small cell using radio resource control (RRC) messaging. After receiving a handover command or detecting radio frequency (RF) loss, the mobile device can try to connect with the target small cell. If the mobile device is unable to connect to the target small cell, the UE can fall back and connect to the macro cell.

PRIORITY

This application claims the benefit of priority from U.S. ProvisionalPatent Application Ser. No. 61/772,493, entitled “Enhanced HetNetMobility Management” and filed on Mar. 4, 2013, which is fullyincorporated herein by reference for all purposes and to the extent notinconsistent with this application.

BACKGROUND

1. Field of the Application

The disclosure is directed to wireless communications and, moreparticularly, to HetNet mobility management in wireless communications.

2. Background of the Disclosure

Wireless communication systems are widely deployed to provide variouscommunication services, such as: voice, video, packet data,circuit-switched info, broadcast, messaging services, and so on. Atypical wireless communication system, or network, can provide multipleusers access to one or more shared resources (e.g., bandwidth, transmitpower, etc.). These systems can be multiple-access systems that arecapable of supporting communication for multiple terminals by sharingavailable system resources. Examples of such multiple-access systemsinclude Code Division Multiple Access (CDMA) systems, Time DivisionMultiple Access (TDMA) systems, Frequency Division Multiple Access(FDMA) systems and Orthogonal Frequency Division Multiple Access (OFDMA)systems.

Generally, a wireless multiple-access communication system cansimultaneously support communication for multiple wireless devices orterminals. In such a system, each terminal can communicate with one ormore base stations via transmissions on the forward and reverse links.The forward link (or downlink) refers to the communication link from thebase stations to the terminals, and the reverse link (or uplink) refersto the communication link from the terminals to the base stations. Thiscommunication link can be established via a single-in-single-out (SISO),single-in-multiple-out (SIMO), multiple-in-signal-out (MISO), or amultiple-in-multiple-out (MIMO) system.

For instance, a MIMO system can employ multiple (N_(T)) transmitantennas and multiple (N_(R)) receive antennas for data transmission. AMIMO channel formed by the N_(T) transmit and N_(R) receive antennas canbe decomposed into N_(S) independent channels, which are also referredto as spatial channels, where N_(S)≦min {N_(T), N_(R)}. Each of theN_(S) independent channels can correspond to a dimension. The MIMOsystem can provide improved performance (e.g., higher throughput and/orgreater reliability) if the additional dimensionalities created by themultiple transmit and receive antennas are utilized.

A MIMO system can support a time division duplex (TDD) and frequencydivision duplex (FDD) systems. In an FDD system, the transmitting andreceiving channels are separated with a guard band (some amount ofspectrum that acts as a buffer or insulator), which allows two-way datatransmission by, in effect, opening two distinct radio links. In a TDDsystem, only one channel is used for transmitting and receiving,separating them by different time slots. No guard band is used. This canincrease spectral efficiency by eliminating the buffer band and can alsoincrease flexibility in asynchronous applications. For example, if lesstraffic travels in the uplink, the time slice for that direction can bereduced, and reallocated to downlink traffic.

Wireless communication systems oftentimes employ one or more basestations that provide a coverage area. A typical base station cantransmit multiple data streams for broadcast, multicast and/or unicastservices, wherein a data stream may be a stream of data that can be ofindependent reception interest to a mobile device. A mobile devicewithin the coverage area of such base station can be employed to receiveone, more than one, or all the data streams carried by the compositestream. Likewise, a mobile device can transmit data to the base stationor another mobile device.

With the proliferation of wireless communications, the use of multipletypes of access nodes may be used across a wireless network or system.Such a network or system is referred to as a HetNet (or heterogeneousnetwork). For example, a wireless system can include larger coveragewide area networks (macrocells, base stations, evolved Node Bs, etc.)that overlay one or more, smaller local area networks (access points,microcells, picocells, femtocells, etc.). HetNets can offer wirelesscoverage in an environment with a wide variety of wireless coveragezones, ranging from an open outdoor environment to office buildings,homes, underground areas, and combinations of these and others. In thisway, a HetNet can be considered a network with complex interoperationbetween macrocell, smaller cells, and in some cases WiFi networkelements used together to provide a mosaic of coverage, with mobiledevice handoff capability between network elements.

In general, HetNet can be deployed to address one or more concerns, twoof which are listed here for illustrative purposes only. First, HetNetcan help increase the coverage area of a typical, or stand-alone, cell.For example, HetNet deployment helps improve coverage in hard to reachareas within the network that cannot be easily or economically served bya macrocell deployment. Second, HetNet can help increase the capacity ofa typical cell. Wireless access network traffic may not be uniformlydistributed throughout a network and there are generally areas within awireless network deployment where subscribers are concentrated in smallgeographical area. An existing macrocell deployment may not be able tomeet the capacity need of these densely subscribed areas. Such denselysubscribed areas can be known as hotspots. In order to address thecapacity need of hotspots, wireless operators are considering the densedeployment of small cells to meet the capacity need. The simultaneousdeployment of small cells and macrocells in hotspot leads to HetNetdeployment. Even though HetNet deployment helps solve capacity problem,it can introduce mobility and interference issues, to name a few.

Therefore, what are needed are techniques to help mitigate at least someof the mobility issues introduced by HetNet deployment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary wireless multiple-access communicationsystem according to certain embodiments;

FIG. 2 illustrates a block diagram of an exemplary mobile device or userequipment (UE) according to certain embodiments;

FIG. 3 illustrates a block diagram of an exemplary enhanced Node B (eNB)or similar mobile communication node (e.g., base station, access point,etc.) according to certain embodiments;

FIG. 4 illustrates an exemplary wireless HetNet configuration accordingto certain embodiments;

FIG. 5 illustrates an exemplary modified HetNet handover flow accordingto certain embodiments;

FIG. 6 illustrates an exemplary modified HetNet handover call flowaccording to certain embodiments;

FIG. 7 illustrates an exemplary modified HetNet handover call flow tosupport radio link failure according to certain embodiments;

FIG. 8 illustrates an exemplary HetNet handover connected use caseaccording to certain embodiments; and

FIG. 9 illustrates an exemplary HetNet handover C-DRX case according tocertain embodiments.

DETAILED DESCRIPTION

The following detailed description is directed to certain sampleembodiments. However, the disclosure can be embodied in a multitude ofdifferent ways as defined and covered by the claims. In thisdescription, reference is made to the drawings wherein like parts aredesignated with like numerals within this application.

This disclosure makes reference to various wireless communicationdevices, such as access point, mobile device, base station, userequipment, Node B, access terminal and eNB. The use of these and othernames is not intended to indicate or mandate one particular device, oneparticular standard or protocol, or one particular signaling directionand is expressly intended to not be limiting of the scope of thisapplication in any way. The use of these and other names is strictly forconvenience and such names may be interchanged within this applicationwithout any loss of coverage or rights.

Various techniques described herein can be used for various wirelesscommunication systems, such as Code Division Multiple Access (“CDMA”)systems, Multiple-Carrier CDMA (“MCCDMA”), Wideband CDMA (“W-CDMA”),High-Speed Packet Access (“HSPA,” “HSPA+”) systems, Time DivisionMultiple Access (“TDMA”) systems, Frequency Division Multiple Access(“FDMA”) systems, Single-Carrier FDMA (“SC-FDMA”) systems, OrthogonalFrequency Division Multiple Access (“OFDMA”) systems, or other multipleaccess techniques. A wireless communication system employing theteachings herein may be designed to implement one or more standards,such as IS-95, cdma2000, IS-856, W-CDMA, TDSCDMA, and other standards. ACDMA network may implement a radio technology such as UniversalTerrestrial Radio Access (“UTRA)”, cdma2000, or some other technology.UTRA includes W-CDMA and Low Chip Rate (“LCR”). The cdma2000 technologycovers IS-2000, IS-95 and IS-856 standards. A TDMA network may implementa radio technology such as Global System for Mobile Communications(“GSM”). An OFDMA network may implement a radio technology such asEvolved UTRA (“E-UTRA”), IEEE 802.11 (“WiFi”), IEEE 802.16 “(WiMAX”),IEEE 802.20 (“MBWA”), Flash-OFDM.®., etc. UTRA, E-UTRA, and GSM are partof Universal Mobile Telecommunication System (“UMTS”). The teachingsherein may be implemented in a 3GPP Long Term Evolution (“LTE”) system,an Ultra-Mobile Broadband (“UMB”) system, and other types of systems.LTE is a release of UMTS that uses E-UTRA. Although certain aspects ofthe disclosure may be described using 3GPP terminology, it is to beunderstood that the teachings herein may be applied to 3GPP (Re199,Re15, Re16, Re17) technology, as well as 3GPP2 (1xRTT, 1xEV-DO Re10,RevA, RevB) technology and other technologies, such as WiFi, WiMAX, WMBAand the like.

Referring now to the drawings, FIG. 1 illustrates an exemplary wirelessmultiple-access communication system 100 according to certainembodiments. In one example, an enhanced Node B (eNB) base station 102includes multiple antenna groups. As shown in FIG. 1, one antenna groupcan include antennas 104 and 106, another can include antennas 108 and110, and another can include antennas 112 and 114. While only twoantennas are shown in FIG. 1 for each antenna group, it should beappreciated that more or fewer antennas may be utilized for each antennagroup. As shown, user equipment (UE) 116 can be in communication withantennas 112 and 114, where antennas 112 and 114 transmit information toUE 116 over downlink (or forward link) 120 and receive information fromUE 116 over uplink (or reverse link) 118. Additionally and/oralternatively, UE 122 can be in communication with antennas 104 and 106,where antennas 104 and 106 transmit information to UE 122 over downlink126 and receive information from US 122 over uplink 124. In a frequencydivision duplex (FDD) system, communication links 118, 120, 124 and 126can use different frequency for communication. In time division duplex(TDD) systems, the communication links can use the same frequency forcommunication, but at differing times.

Each group of antennas and/or the area in which they are designed tocommunicate can be referred to as a sector of the eNB or base station.In accordance with one aspect, antenna groups can be designed tocommunicate to mobile devices in a sector of areas covered by eNB 102.In communication over downlinks 120 and 126, the transmitting antennasof eNB 102 can utilize beamforming in order to improve thesignal-to-noise ratio of downlinks for the different UEs 116 and 122.Also, a base station using beamforming to transmit to UEs scatteredrandomly through its coverage causes less interference to mobile devicesin neighboring cells than a base station transmitting through a singleantenna to all its UEs. In addition to beamforming, the antenna groupscan use other multi-antenna or antenna diversity techniques, such asspatial multiplexing, spatial diversity, pattern diversity, polarizationdiversity, transmit/receive diversity, adaptive arrays, and the like.

FIG. 2 illustrates a block diagram 200 of an exemplary mobile device oruser equipment (UE) 210 according to certain embodiments. As shown inFIG. 2, UE 210 may include a transceiver 210, an antenna 220, aprocessor 230, and a memory 240 (which, in certain embodiments, mayinclude memory in a Subscriber Identity Module (SIM) card). In certainembodiments, some or all of the functionalities described herein asbeing performed by mobile communication devices may be provided byprocessor 230 executing instructions stored on a computer-readablemedium, such as the memory 240, as shown in FIG. 2. Additionally, UE 210may perform uplink and/or downlink communication functions, as furtherdisclosed herein, via transceiver 210 and antenna 220. While only oneantenna is shown for UE 210, certain embodiments are equally applicableto multi-antenna mobile devices. In certain embodiments, UE 210 mayinclude additional components beyond those shown in FIG. 2 that may beresponsible for enabling or performing the functions of UE 210, such ascommunicating with a base station in a network and for processinginformation for transmitting or from reception, including any of thefunctionality described herein. Such additional components are not shownin FIG. 2 but are intended to be within the scope of coverage of thisapplication.

FIG. 3 illustrates a block diagram 300 of an exemplary enhanced Node B(eNB) 310 or similar mobile communication node (e.g., base station,access point, etc.) according to certain embodiments. As shown in FIG.3, eNB 310 may include a baseband processor 310 to provide radiocommunication with mobile handsets via a radio frequency (RF)transmitter 340 and RF receiver 330 units coupled to the eNB antenna320. While only one antenna is shown, certain embodiments are applicableto multi-antenna configurations. RF transmitter 340 and RF receiver 330may be combined into one, transceiver unit, or duplicated to facilitatemultiple antenna connections. Baseband processor 320 may be configured(in hardware and/or software) to function according to a wirelesscommunications standard, such as 3GPP LTE. Baseband processor 320 mayinclude a processing unit 332 in communication with a memory 334 toprocess and store relevant information for the eNB and a scheduler 336,which may provide scheduling decisions for mobile devices serviced byeNB 310. Scheduler 336 may have some or all of the same data structureas a typical scheduler in an eNB in an LTE system.

Baseband processor 330 may also provide additional baseband signalprocessing (e.g., mobile device registration, channel signal informationtransmission, radio resource management, etc.) as required. Processingunit 332 may include, by way of example, a general purpose processor, aspecial purpose processor, a conventional processor, a digital signalprocessor (DSP), a plurality of microprocessors, one or moremicroprocessors in association with a DSP core, a controller, amicrocontroller, Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine. Some or all of the functionalitiesdescribed herein as being provided by a mobile base station, a basestation controller, a node B, an enhanced node B, an access point, ahome base station, a femtocell base station, and/or any other type ofmobile communications node may be provided by processing unit 332executing instructions stored on a computer-readable data storagemedium, such as the memory 334 shown in FIG. 3.

In certain embodiments, eNB 310 may further include a timing and controlunit 360 and a core network interface unit 370, such as are shown inFIG. 3. Timing and control unit 360 may monitor operations of basebandprocessor 330 and network interface unit 370, and may provideappropriate timing and control signals to these units. Network interfaceunit 370 may provide a bi-directional interface for eNB 310 tocommunicate with a core or back-end network (not shown) to facilitateadministrative and call-management functions for mobile subscribersoperating in the network through eNB 310.

Certain embodiments of the base station 310 may include additionalcomponents responsible for providing additional functionality, includingany of the functionality identified herein and/or any functionalitynecessary to support the solution described herein. Although featuresand elements are described in particular combinations, each feature orelement can be used alone without the other features and elements or invarious combinations with or without one or more features and elements.Methodologies provided herein may be implemented in a computer program,software, or firmware incorporated in a computer-readable storage medium(e.g., memory 334 in FIG. 3) for execution by a general purpose computeror a processor (e. g., processing unit 332 in FIG. 3). Examples ofcomputer-readable storage media include read only memory (ROM), randomaccess memory (RAM), digital registers, cache memory, semiconductormemory devices, magnetic media such as internal hard disks, magnetictapes and removable disks, magneto-optical media, and optical media suchas CDROM disks, digital versatile disks (DVDs), and so on.

FIG. 4 illustrates an exemplary wireless HetNet configuration 400according to certain embodiments. As shown in FIG. 4, a mobile device(handset, UE, laptop, tablet, etc.) 430 is within the coverage area ofHetNet 400. HetNet 400 can include multiple network coverage pieces. Forexample, the largest coverage area can be a macrocell (eNB, basestation, etc.) 410A. Within (or partially within) macrocell 410Acoverage area, there can concurrently exist one or more sub-cell orsmall cell coverage areas. As shown, small cells 410B, 410C are within(or partially within) macrocell 410A and at least partially overlap eachother. Each cell 410 can also include some sort of network access device420A, 420B and 420C, such as a base station or access point. Eachnetwork access device 420 can communicate with one or more mobiledevices 430, as well as with a core network 440. Not shown are possibleintermediate network components or system elements that may be between anetwork access device 420 and the core network 440. In certainembodiments applicable to this application, mobile device 430 can bemoving within macrocell 410A and moving out of small cell 410B coveragearea and into small cell 410C coverage area. In this way, the mobiledevice 430 could possibly communicate with all three cells: macrocell410A small cell 410B and/or small cell 410C.

In certain embodiments, a HetNet system can include many small cellsoverlapping, or within, a macrocell. Mobile devices, such as userequipment (UE), within the range of a small cell inside a HetNet aretypically able to communicate with both the overlay macrocell and anearby small cell. In HetNet, when a UE is connected to a small cell andis moving fast or in discontinuous reception (DRX) mode, it may finditself at the edge of the connected cell and it may not have enough timeto perform active mode handover to the target small cell. In such acase, the active mode handover may fail and the UE can experience radiolink failure (RLF). Upon detecting radio link failure, the UE attempt torecover from it by performing radio resource control (RRC)reestablishment procedure.

In LTE, DRX mode can be enabled in both RRC_IDLE and RRC_CONNECTEDstates. In the RRC_IDLE state, the UE is registered with the evolvedpacket system (EPS) mobility management (EMM) but does not have anactive session. In this state, the UE can be paged for downlink (DL)traffic. The UE can also initiate uplink (UL) traffic by requesting RRCconnection with the serving eNB. In LTE, DRX mode can also be enabled inRRC_CONNECTED state. In the RRC_CONNECTED state, DRX mode is enabledduring the idle periods during the packet arrival process. When thereare no outstanding/new packets to be transmitted/received, the eNB/UEmay initiate the DRX mode.

However, it is likely that a UE can end up on a cell that does not havea prepared UE context, because typical small cells are single sector innature. This is in contrast to a mobile device moving between sectors ofa multi-sector macrocell, where the controller of the macrocell canmaintain eNodeB specific RRC states for the UE. So, in a macrocell, eventhough the UE moves from one sector to another sector, the eNodeB usesthe same RRC state for the UE from one sector to the next. In suchscenarios, RRC reestablishment can fail and the UE will be required toperform an attach or service request procedure to recover from the radiofrequency (RF) loss. The attach or service request procedure canadversely impact the performance of the UE.

FIG. 5 illustrates an exemplary modified HetNet handover flow 500according to certain embodiments. In HetNet deployment, assuming a smallcell is the serving or source cell for the mobile device 510 and thatthe UE is moving from one small cell to another small cell (and both ofwhich are within the coverage area of a macrocell), the source cell willprepare both a target small cell as well as the macrocell duringhandover preparation phase 520. This preparation phase can beaccomplished at handover or long before handover (i.e., can be“infinite”), so that it does not have to be accomplished at handover.For example, in LTE, the source cell can inform the UE of the targetsmall cell and that the macrocell is using system information blocks(SIBs) 4 and/or 5, which can provide neighbor information and RRCmessages 530.

Upon receiving a handover command from source cell, the UE can firstattempt to perform handover on the target small cell 540. If successful550, then the handover is complete 560. If unsuccessful, 550, then theUE can fall back to the macrocell and perform handover with themacrocell 570. Upon detecting RF loss, UE can attempt to perform RRCreestablishment procedure with the macrocell. Upon successful handoveror re-establishment with the macrocell, the macrocell can offload the UEto the nearby small cell to avoid overloading the macrocell 580.

In summary, a UE can perform the following in an attempt to improvehandover success rate for the UE in a HetNet deployment. The source eNBcan prepare macro eNodeB in addition to a target small cell as handovercandidates during handover decision making The UE is informed about thetarget small cell and eNodeB using RRC messaging. After receivinghandover command or detecting RF loss, the UE can try to connect withthe target small cell. If UE is unable to connect to the target smallcell, the UE will connect to the macro eNodeB. The eNodeB on which theUE will connect can inform MME using Si messaging.

Existing LTE X2/S 1 based handover mechanisms, which are incorporatedherein by reference, can be used to prepare both the macrocell as wellas a target small cell. The candidate macrocell can be prepared forinfinite duration where the source will have to cancel the preparationif it does not want the UE to associate with the prepared cells, or caninclude a large timer value (e.g. based on data or speech connection orcall duration, for example hours or half an hour after disconnect). FIG.6 illustrates an exemplary modified HetNet handover call flow accordingto certain embodiments. FIG. 7 illustrates an exemplary modified HetNethandover call flow to support radio link failure according to certainembodiments.

Certain embodiments of this disclosure can include a method to reducehandoff failures in HetNet deployment by preparing both target smallcell and overlay macrocell. The source small cell can proactivelyprepare both overlay macrocell and target small cell during handoverpreparation before sending a handover command to a UE. The source smallcell can inform the UE, using an RRC message, of the identity of theprepared handover candidates. The UE can first attempt to completehandover procedure with the target small cell. If unable to completehandover, it can fall back to the prepared macrocell. In case of radiolink failure, the UE can fall back to the prepared macrocell. As soon asUE is able to successfully perform RRC reestablishment with targetmacrocell, the macrocell can offload the UE to a nearby small cell usingstandard procedures.

With increased UE speed or mobility, frequent handovers may occur in aHetNet deployment due to the potential of larger number of small cellsizes. This could lead to handover performance degradation, increasedradio link failure (RLF) and RRC re-establishment procedures, andunnecessary handovers or a “Ping-Pong” effect. In certain embodiments,the eNB might consider the mobility or mobility state (as discussedbelow) of a UE to decide if the eNB needs to prepare a macro eNodeB inadditional to small cells to support handover failure and/or RF losssituations.

In LTE, any data transmission requires that the UE be in high power RRCconnected state. With all data applications there are often moments whenno data is being sent or received and during those moments, using aconnected state discontinuous reception (DRX) mode can save energy(i.e., battery life of the UE). Connected state DRX (C-DRX or cDRX)cyclically shuts down and wakes up the UE receiver circuits in order tosave energy, with the goal of not adversely affecting the performance ofthe active data application and/or its ability to effectivelycommunicate with the serving cell.

In certain embodiments, the UE can report to the eNB its mobility state.The UE can determine its mobility state in a myriad of ways. Forexample, the UE can use on-device motion sensors to determine itsmobility state. The UE may also use one or more other aspects todetermine its mobility state, such as: relative eNB signal strength, GPSmovement, Doppler parameters, and so on. This mobility state can be assimple as high, medium, low, or can provide more detail and granularity.This reporting can be done through a medium access control (MAC) controlelement (CE) or during a UE measurement report as configured by the eNBthrough RRC. Also, new, non-standard messaging may be used.Alternatively, the eNB can estimate the UE's mobility state (high,medium . . . ) through RRC.

The eNB can then decide to change the RRC inactivity timer based atleast in part on the UE's mobility state (i.e., shorten the timer forhigher speed UEs). Typically, the standard timer might be set at 1.28seconds. This can allow the UE to move quickly to idle state and avoidunnecessary handovers, due to crossing cells, and/or the Ping-Pongeffect. The handover (H/O) avoidance can happen because, in the LTEstandard, no H/O will occur in idle state (which can instead use cellreselection). The eNB will also have the UE configured with a differentset of parameters for shortening the time between RLF to RRCre-establishment. For example, the shortened time might be 100milliseconds (ms) instead of 300 ms. These parameters can be effectiveonly if the mobility state at the UE has crossed a certain threshold.

In C-DRX, the eNB can have different parameter configurations dependingon the UE speed (e.g., or UE mobility state). For example, if a UE is ina high mobility state, then the C-DRX off period can be shortened (e.g.,from 320 ms down to 1-16 ms) to allow the UE to make more accuratereference signal received power (RSRP)/reference signal received quality(RSRQ)/radio link monitoring (RLM) measurements (e.g., as defined in LTEstandard 36.214, among others, which is fully incorporated herein byreference) but also to be able to discover small cells more frequently.

FIG. 8 illustrates an exemplary HetNet handover connected use case 800according to certain embodiments. In case there is no data transferbetween the UE and eNB for a period of time specified by RRC inactivitytimer, the UE will move to idle mode. In idle mode, there is nohandover, the UE will do a cell re-selection which is less stringentthan H/O.

FIG. 9 illustrates an exemplary HetNet handover C-DRX case 900 accordingto certain embodiments. In C-DRX, the UE and eNB can exchange data inthe C-DRX on duration and not in the C-DRX sleep phase. If/when the eNBdetects that the UE is in a high mobility state, it can change the C-DRXconfiguration (through RRC reconfiguration) by shortening the C-DRXsleep phase.

Once in idle mode, per the LTE standards, the UE will camp on thestrongest cell and every 1.28 seconds, the UE reads pages. Also,periodically, the UE reads cell measurements to determine which cellmight be strongest, along with other cell info. According to certainembodiments, the UE can camp on the macrocell during idle mode to avoidmultiple cell reselections even if a small cell may have a strongersignal. This means that a mechanism to identify the cell (small vs.macro) is in place. SIB4 and SIB4 biasing information can be used todecide if an eNB is macro or node. The UE also will synchronize with thesmall cell (i.e., will read primary and secondary synchronizationsignals (PSS/SSS)/master and system information blocks (MIB/SIB) of thestrongest small cell). In this way, the UE can camp on two cells (themacrocell and a small cell). When the UE is paged (i.e., a downlinkprocess), it will first try to answer it in the macrocell (since it iscamping on it). If it fails on the macrocell, it will try on the smallcell, since it has the information ready, reducing the time needed toestablish a connection with the small cell.

If the UE needs to transfer data to the eNB (i.e., an uplink process),the UE can check the amount of data (e.g., the information is availableat the buffer status), if the amount of data is small (e.g., SMS, email,etc.), the UE will connect to the macrocell, otherwise, if the amount ofdata is large (e.g., streaming video, some web browsing, location-basedservices, etc.) the data can go over the small cell. If the datatransfer is for voice, then the UE can connect to the strongest cell, orperhaps, try the macrocell first and then the small cell second.

For UE paging, when in idle mode, the UE can camp on the macrocell. TheUE will identify the macrocell by reading the biasing information onSIB4/SIB5. The UE can also read the PSS/SSS/MIB/SIB of the strongestsmall cell. When a page is received the UE can first try to attach tothe macrocell; in case of failure, then the UE will attach to the smallcell. The UE can also use its mobility state to help determinewhen/whether to attach to a prepared small cell versus a prepared macrocell. For example, if the UE determines it is in a high mobility state,it can choose to attach to a macro cell instead of a small cell to helpprevent frequent handovers and/or the “Ping-Pong” effect. Likewise, ifthe UE determines it is in a low mobility state, it can choose the smallcell.

Those of ordinary skill in the art would understand that information andsignals may be represented using any of a variety of differenttechnologies and techniques. For example, data, instructions, commands,information, signals, bits, symbols, and chips that may be referencedthroughout the above description may be represented by voltages,currents, electromagnetic waves, magnetic fields or particles, opticalfields or particles, or any combination thereof

Those of ordinary skill would further appreciate that the variousillustrative logical blocks, modules, and algorithm steps described inconnection with the examples disclosed herein may be implemented aselectronic hardware, firmware, computer software, middleware, microcode,or combinations thereof. To clearly illustrate this interchangeabilityof hardware and software, various illustrative components, blocks,modules, circuits, and steps have been described above generally interms of their functionality. Whether such functionality is implementedas hardware or software depends upon the particular application anddesign constraints or preferences imposed on the overall system. Skilledartisans may implement the described functionality in varying ways foreach particular application, but such implementation decisions shouldnot be interpreted as causing a departure from the scope of thedisclosed methods.

The various illustrative logical blocks, components, modules, andcircuits described in connection with the examples disclosed herein maybe implemented or performed with a general purpose processor, a digitalsignal processor (DSP), an application specific integrated circuit(ASIC), a field programmable gate array (FPGA) or other programmablelogic device, discrete gate or transistor logic, discrete hardwarecomponents, or any combination thereof designed to perform the functionsdescribed herein. A general purpose processor may be a microprocessor,but in the alternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theexamples disclosed herein may be embodied directly in hardware, in oneor more software modules executed by one or more processing elements, orin a combination of the two. A software module may reside in RAM memory,flash memory, ROM memory, EPROM memory, EEPROM memory, registers, harddisk, a removable disk, a CD-ROM, or any other form or combination ofstorage medium known in the art. An example storage medium is coupled tothe processor such that the processor can read information from, andwrite information to, the storage medium. In the alternative, thestorage medium may be integral to the processor. The processor and thestorage medium may reside in an Application Specific Integrated Circuit(ASIC). The ASIC may reside in a wireless modem. In the alternative, theprocessor and the storage medium may reside as discrete components inthe wireless modem.

The previous description of the disclosed examples is provided to enableany person of ordinary skill in the art to make or use the disclosedmethods and apparatus. Various modifications to these examples will bereadily apparent to those skilled in the art, and the principles definedherein may be applied to other examples and additional elements may beadded.

What is claimed is:
 1. A system for mobility management in a HetNetwireless communication system comprising: a source small cell incommunication with a mobile device, wherein the source small cell isconfigured to: proactively prepare an overlay macrocell and a targetsmall cell during handover preparation before sending a handover commandto the mobile device; inform the mobile device of the identities of theprepared overlay macrocell and the prepared target small cell usingradio resource control (RRC) messaging; and send the handover command tothe mobile device.
 2. The system of claim 1, wherein the mobile deviceis configured to: receive the RRC messaging from the source small cell;receive the handover command from the source small cell; first attemptto complete handover with the prepared target small cell, and if thefirst attempt fails, then fall back to complete handover with theprepared overlay macrocell.
 3. The system of claim 2, wherein the mobiledevice is further configured to: detect a radio link failure; firstattempt to perform RRC reestablishment with the prepared overlaymacrocell; and after reestablishment success, perform handover from theoverlay macrocell to a nearby small cell.
 4. The system of claim 1,wherein the mobile device is configured to: receive the RRC messagingfrom the source small cell; detect a radio link failure; first attemptto perform RRC reestablishment with the prepared overlay macrocell; andafter reestablishment success, perform handover from the overlaymacrocell to a nearby small cell.
 5. A method for mobility management ina HetNet wireless communication system comprising: at a source smallcell, which is in communication with a mobile device: proactivelypreparing an overlay macrocell and a target small cell during handoverpreparation before sending a handover command to the mobile device;informing the mobile device of the identities of the prepared overlaymacrocell and the prepared target small cell using radio resourcecontrol (RRC) messaging; and sending the handover command to the mobiledevice.
 6. The method of claim 5, wherein, at the mobile device:receiving the RRC messaging from the source small cell; receiving thehandover command from the source small cell; first attempting tocomplete handover with the prepared target small cell, and if the firstattempt fails, then falling back to complete handover with the preparedoverlay macrocell.
 7. The method of claim 6, wherein, at the mobiledevice: detecting a radio link failure; first attempting to perform RRCreestablishment with the prepared overlay macrocell; and afterreestablishment success, performing handover from the overlay macrocellto a nearby small cell.
 8. The method of claim 5, wherein, at the mobiledevice: receiving the RRC messaging from the source small cell;detecting a radio link failure; first attempting to perform RRCreestablishment with the prepared overlay macrocell; and afterreestablishment success, performing handover from the overlay macrocellto a nearby small cell.
 9. A method for mobility management using amobile device in a HetNet wireless communication system comprising:receiving radio resource control (RRC) messaging that identifies anoverlay macrocell and a target small cell that have been proactivelyprepared during handover preparation by a source small cell; receiving ahandover command; attempting to complete handover with the preparedtarget small cell; and if attempting to complete handover with theprepared target small cell fails, falling back to the prepared overlaymacrocell.
 10. The method of claim 9, comprising: detecting a radio linkfailure; attempting to perform RRC reestablishment with the preparedoverlay macrocell; and after reestablishment success, performinghandover from the overlay macrocell to a nearby small cell.
 11. Anapparatus for mobility management using a mobile device in a HetNetwireless communication system comprising: means for receiving radioresource control (RRC) messaging that identifies an overlay macrocelland a target small cell that have been proactively prepared duringhandover preparation by a source cell; means for receiving a handovercommand; means for attempting to complete handover with the preparedtarget small cell; and if the means for attempting to complete handoverwith the prepared target small cell fails, means for falling back to theprepared overlay macrocell.
 12. The apparatus of claim 11, comprising:means for detecting a radio link failure; means for attempting toperform RRC reestablishment with the prepared overlay macrocell; andafter reestablishment success, means for performing handover from theoverlay macrocell to a nearby small cell.
 13. A system for mobilitymanagement in a HetNet wireless communication system comprising: amobile device in a connected mode with a source cell, wherein the sourcecell determines a mobility state about the mobile device, wherein themobility state includes at least one lower mobility state and at leastone higher mobility state; wherein the source cell decides a shortenedradio resource control (RRC) inactivity timer if the mobile device is inthe at least one higher mobility state; and wherein the source cellconfigures the mobile device with a different set of parameters for theshortened RRC inactivity timer, given that the mobile device has crosseda certain threshold.
 14. The system of claim 13, wherein the mobiledevice reports a mobility state to the source cell.
 15. The system ofclaim 14, wherein the mobile device reports to the source cell throughat least one of a media access control (MAC) control element (CE) and amobile device measurement report, as configured by the source cellthrough radio resource control (RRC).
 16. The system of claim 13,wherein the source cell estimates the mobility state of the mobiledevice.
 17. A method for mobility management for a source cell in aHetNet wireless communication method comprising: determining a mobilitystate for a mobile device, wherein the mobility state includes at leastone lower mobility state and at least one higher mobility state;deciding a shortened radio resource control (RRC) inactivity timer ifthe mobile device is in the at least one higher mobility state; andsending mobile device configuration, wherein the configuration includesa different set of parameters for the shortened RRC inactivity timer,given that the mobile device has crossed a certain threshold.
 18. Themethod of claim 17, wherein the mobility state is received from themobile device.
 19. The method of claim 18, wherein the mobility state isreceived through at least one of a media access control (MAC) controlelement (CE) and a mobile device measurement report, as configured bythe source cell through radio resource control (RRC).
 20. The method ofclaim 17, wherein the mobility state is estimated by the source cell.